# Questions about light, theory of relativity

1. Jan 10, 2009

### gabriel4786

Ok, I don't know all there is to know about the theory of relativity, so with that said, hopefully someone can answer this question!
How can light always move away from its source at a constant speed speed? In other words, lets say the speed of light is 5+. That means that light is always 5 faster than you. Say you are 0. That means that light is 5.
Now you are 3. Then light is 8.
You are still going three, but this time there is another person going 2. To that person, light is 7. But to you, it is 8! This is what confuses me. How is it possible that light is one speed, but another speed at the same time? Would not 3 and 2 also experience different speeds of time? Is there any writing out there that explains why time is not fixed and constantly uniform, and varies in speed based on velocity of movement through space? What happens if light suddenly vanished from existence, would time still exist?

Another question, is what direction does light travel? Can it move backwards, or just change direction? In the vastness of space, there is no up, down, forward or backward, so how does light know to move in what we perceive as forward, or in other words, away from its source?

Another question, if you were to zoom into the individual photons that make up a beam of light, would a photon 5 photons behind another be moving slightly slower in time? Or do all of the photons travel at the same rate, hence they are in the same time? In other words, if you were riding a photon in a beam of light, would the photon ahead of you be traveling away from you at the speed of light, or the same speed?

This should be an easy one to explain, if photons have a mass of 0, how is it possible that the extreme gravity produced by a black hole is able to pull light in? Gravity affects mass, correct? So how is gravity able to grab hold of a massless particle? Or does light just follow the displacement of the fabric of space?

I appreciate any replies and I'm aware that some if not all of what I talk about has already been answered! Thanks for your understanding that I'm really not educated in this field and what I'm discussing are just thoughts I've asked myself over the years as an introspective individual. Thanks again!

2. Jan 11, 2009

### Staff: Mentor

No, it means that light is always 5 units with respect to you (that is: as measured by you). It happens that it always moves at 5 units with respect to anybody.
No. Now you are viewing things from someone else's frame. That observer sees you move at 3 and the light move at 5 (all measurements with respect to him).
No. All will see light move at the same speed with respect to them.
It's not. In fact, just the opposite.
If you are moving with respect to me, I would measure your clocks to be running slowly compared to mine. Of course, you would observe the same thing happening to my clocks as compared to your own. Everyone sees their own clocks running normally, of course.

There are many, many accessible treatments of basic relativity.